1. Field of the Invention
The invention relates to a control apparatus and control method for an internal combustion engine having a supercharging device.
2. Description of the Related Art
For example, Japanese patent application publication No. 05-296070 describes an apparatus that the time period during which both the intake valve and the exhaust valve are open (hereinafter, referred to as “valve-overlap duration”) is reduced when the engine is operating in a supercharging region at a low engine speed. This apparatus is equipped with a variable valve timing mechanism and controls the closing valve timing of the intake valves in a low engine speed range to be retarded as compared in a high engine speed range. Generally, because the valve-overlap duration is longer in a low engine speed range than in a high engine speed range, the injected fuel in the low engine speed range may partially flow out to the exhaust ports without being combusted (this will be referred to as “outflow of injected fuel” where necessary). Particularly, the outflow of injected fuel may show up in engines having a supercharger. Therefore, by reducing the valve-overlap duration in the low engine speed range, the fluctuation of air-fuel ratio due to the outflow of injected fuel may be suppressed accordingly, which improves the fuel efficiency.
However, if the valve timing for closing the intake valves is retarded in order to reduce the valve overlap duration in the low engine speed range, the intake air in the cylinder may flow back into the intake port (this will be referred to as “backflow of intake air” where necessary) because the inertia force of the intake air is weak in the low engine speed range. Thus, the packing efficiency of intake air in the cylinder may decrease, and thereby the engine output may decrease particularly when the engine load is high.